The present invention relates generally to an optical inspection system for detecting printing flaws in items where high quality printing is desired such as for bank notes, postage stamps, stock certificates and the like.
Bank notes and other printer paper of value such as postage stamps, stock certificates and the like are normally printed to very high quality standards for two principal reasons. First, the increased cost associated with high quality is justified by the value of the end product, and second, the high quality standards discourage potential counterfeiters. Despite all precautions, however, a small percentage of the printed product is produced with printing defects. Presently, such printing defects are discovered by manual inspection which is an expensive process and vulnerable to the subjective judgements and human frailties of the inspector. It is desirable, therefore, to substitute high speed automatic inspection for the present manual process.
The following discussion and description of the invention will concentrate on the problems and the solution to the problems relating to inspection of bank notes. However, those of skill in the art will recognize that the problems and solutions as they relate to bank notes are also common to the problems of high quality printing for postage stamps, stock certificates and other paper of value. Accordingly, the concepts of the present invention are applicable to any environment where high quality standards must be maintained in a printed product.
Ideally, it is desirable to perform inspection by making a point-by-point comparison between a test note to be inspected and a master note. The presence of a defect would then be determined by establishing a threshold on the difference resulting from each comparison. In reality, the "points" being compared are small finite areas approximately equal to the dimensions of the smallest speck that can be seen by the unaided human eye.
The approach adopted in accordance with the present invention assumes the comparison is between equivalent points. The procedure is analogous to a microscopic equivalent of the manual process in which the inspector compares eye for eye, nose for nose, etc., in the portrait area of two currency notes to determine a level of similarity. This technique requires the two notes to be properly registered while they are being viewed.
A major problem to be overcome before the inspection technology can be successful arises from the dimensional instability of the paper used for bank notes. This dimensional instability is also found in the paper used for other forms of high quality printing. Because of this paper instability, it is impossible to bring the entire test note into registration simultaneously with the reference note. Specifically, it has been determined that even if some portion of each note is brought into exact registration with the other note, the notes could be out of registration in other areas by as much an order of magnitude more than the dimension of the incremental areas being compared. Accordingly, a major objective of the present invention is to continuously and automatically maintain registration between the test note and the master note against which the test note is compared so as to compensate for paper instability.
In view of the foregoing objective, it is necessarily axiomatic that the present invention must be able to continuously measure registration error in two dimensions between two similar images. The apparatus must be capable of performing the electronic equivalent of a manual procedure in which one dithers two transparencies along two orthogonal axes to determine the best fit.
In addition to being able to detect a registration error, it is necessary for the system to be able to correct registration errors in two dimensions so that pixels (picture elements) on the test note may be compared to corresponding pixels on a reference note in real time.
It is highly desirable to utilize digital electronics in a system according to the present invention, however, a digital system operates in discrete step sizes so that implementation of a tracking error corrector results in what is referred to as a quantization error which is an ultimate limiting factor on tracker performance. For example, if the step size is one pixel, the minimum quantization error would be one half pixel. This occurs because any attempt to correct an error of less than one half pixel would result in creating an error of greater than one half pixel and of the opposite sign. In general, the minimum quantization error is equal to one half the step size of the correction. It is, therefore, a further objective of the present invention to incorporate a mechanism for minimizing the step size of the tracking error correction as well as prevent tracking error correction when such correction will produce a larger tracking error than the error sought to be corrected.
A further problem associated with currency inspection is to ensure that substantially all of the test note is scanned and compared with a reference note. Accordingly, it is yet a further objective of the present invention to provide a mechanism for quickly achieving initial registration between the test note and the reference note so that even the very first part of each note tested is compared with the reference note.
Since every system has some low level noise, it is a further objective of the present invention to provide an optical comparator which has a low gain when viewing areas of a printed sheet having no detail and high gain when viewing areas of a printed sheet having maximum detail thereby minimizing the effect of system noise.